1. Field of the Invention
The invention relates to a digital to analog converter, and more particularly to a digital to analog converter with reduced number of switches.
2. Description of the Related Art
Digital to analog converters are widely used in mixed-mode systems where the converter acts as an interface between the digital signal processing and analog signal processing components of mixed-mode systems.
FIG. 1 is a circuit diagram of a conventional 23-to-2 digital to analog converter. The converter is controlled by a 3-bits input signal, and the output voltages V0 and V1 are determined according to the input signal. The converter requires 3×23+23 switches for implementation. In other words, a conventional 2N-to-2 digital to analog converter requires N×2N+2N switches for implementation. If N is 10, the number of digital to analog converters is large and the layout area accordingly is large.
FIG. 2 is a schematic diagram of a general architecture of a two stage N to 1 digital to analog converter. The two stage digital to analog converter comprises a first digital to analog converter (DAC) 21 controlled by (n−2) bits and a second stage with four switches controlled by the LSB (least significant bit) 2 bits. The DAC 21 outputs two voltages according to the most (n−2) bits, and then the LSB 2 bits control the four switches to output the output voltage Vout. The traditional architecture DAC 21 requires a large number of switches and this occupies large layout area. Therefore, the invention provides architectures of DACs that require fewer switches.